1. Field of the Invention
The invention relates to a machine for grinding large balls, in particular of natural or artificial stone, comprising a dish-shaped grinding tool which is arranged vertically above the ball blank mounted on rolling bodies, at least one of which is driven, and a means for producing the grinding pressure between the ball blank and the grinding tool.
2. Description of the Prior Art
Whereas for grinding relatively small balls with a diameter of a few millimeters up to a few centimeters various techniques are available, the grinding of large balls, in particular of natural or artificial stone, still presents great difficulties. An example is a granite ball having a diameter of about 10 cm to 3 m as is required for so-called "floating balls", i.e. a large ball mounted in a dish by means of a liquid film, in particular a water film, and lifted by the hydrostatic liquid pressure; such a "floating ball" turns very easily because of the extremely low frictional forces between the dish and ball.
A machine for grinding large balls of the type set forth is known from U.S. Pat. No. 2,998,676 and comprises a dish-shaped grinding tool which is arranged vertically above a ball blank mounted on rolling bodies, of which at least one is driven. The necessary grinding pressure between the ball blank and the grinding tool is produced by a weight, the grinding tool being mounted on a pivotable lever arm.
A disadvantage with such a machine is the complex construction because the interchangeable grinding disc serving as grinding tool must be displaced by means of an adjustment mechanism parallel to the two rolling bodies supporting the ball blank; the guiding of the grinding disc must be with high accuracy and at the same time, however, compensate any deviations, inevitable in practice, in the diameter of the ball blank from the desired value. With relatively small objects, for example balls for ball bearings, this does not involve any difficulties. However, it is not readily possible to use the known machine to grind balls of natural or artificial stone having a diameter of 1 to 3 m with the desired precision of the order of magnitude of about same 100ths mm.